1. Field of the Invention
The present invention relates to an ATM (asynchronous transfer mode) cell transfer apparatus, and more particularly to an ATM cell transfer apparatus which uses, for an upper layer, AAL5 (ATM Adaptation Layer Type5) which is defined by ITU-T Recommendation I.363 or AAL3/4 (ATM Adaptation Layer Type3/4).
2. Description of the Related Art
A conventional ATM communication system has a high-speed transfer function of ATM cells. In addition, the conventional ATM communication system has to quickly eliminate an overload state of a transmission line when the overloaded state or a congestion state is generated on the transmission line, as shown in, for example, Japanese Laid Open Patent Disclosure (JP-A-Heisei 7-58748).
FIG. 1 is a block diagram illustrating an example of the conventional ATM communication system. Referring to FIG. 1, the conventional ATM communication system is composed of a connection attribute table 9, a switch control unit 10, an ATM switch unit 11, a routing table 12, input transmission line accommodating units 13 and 14, output transmission line accommodating units 15 and 16, congestion detecting units 17 and 18, input transmission lines 21 and 22, output transmission lines 23 and 24, and a ATM cell accumulating unit 29.
In this ATM communication system, the ATM cells which arrive through the input transmission lines 21 and 22 are inputted to the ATM switch unit 11 through the input transmission line accommodating units 13 and 14, respectively. The routing table 12 is referred to based on a connection identifier of each of the arriving ATM cells, and the ATM switch unit 11 sends out the ATM cell to the output transmission line 23 or 24 which corresponds to the connection identifiers. However, when the information which indicates that the ATM cell is to be discarded is stored in the routing table 12, the ATM cell is discarded.
In this ATM communication system, when the load becomes heavy so that congestion occurs, congestion generating information is notified to the switch control unit 10 from one of the congestion detecting units 17 and 18 of the output transmission line accommodating units 15 and 16. The switch control unit 10 selects the connection which has lower importance and which elimination of the congestion can be expected, based on information of connection importance which is stored in the connection attribute table 9. Then, the switch control unit 10 regulates traffic to eliminate the congestion by discarding a part or all of the ATM cells or temporarily saving the ATM cells in the ATM cell accumulating unit 29.
FIG. 2 is a diagram illustrating a protocol stack in the ATM communication system which uses, for the upper layer, AAL5 which is defined in ITU-T Recommendation I.363 (published in November, 1993). In the protocol stack, a protocol layer is separated into two layers of an AAL5 layer and an ATM layer. Further, the AAL5 layer is separated into 3 layers of SSCS (Service Specific Convergence Sublayer), CPCS (Common Part Convergence Sublayer), and SAR (Segmentation And Reassembly) from the uppermost layer.
The operation in each layer in a transmission terminal will be first described with reference to FIG. 3. Referring to FIG. 3, in the SSCS layer, header information and trailer information which are defined by the service of the upper layer of AAL5 are added to SSCS SDU (SSCS Service Data Unit) to generate SSCS PDU (SSCS Protocol Data Unit). The SSCS PDU is sent to the CPCS layer. In the CPCS layer, the trailer information for AAL5 such as an error correction code CRC-32 and a code for checking data length which are defined in ITU-T is added to generate CPCS PDU. The CPCS PDU is sent to the SAR layer. In the SAR layer, the data from the CPCS layer is divided in units of 48 bytes to produce ATM cells and is sent to the ATM layer. In the ATM layer, an ATM cell header such as connection identification information is added to the 48-byte data from the SAR layer and is transmitted to a lower layer. In the case that the ATM cell header is added, an AUU bit of the ATM cell header is set to "1" when the data is the last SAR PDU, and the AUU bit is set to 0, otherwise.
At a terminal on the reception side, the above-mentioned operations of the respective layers in the terminal on the transmission side are executed inversely. A transfer error detecting mechanism such as an error detection code is embedded in each layer and the terminal on the reception side is constructed in such a manner that it is possible to detect an error in the lower layer. For example, when one of the ATM cells is discarded while the ATM cells are transferred, the data is processed just as it is in the ATM layer and the SAR layer. However, a part of CPCS PDU which is produced in the terminal on the reception side is missing so that the data length check error and the CRC-32 error are detected in the CPCS layer. Because the data of the discarded ATM cell can not be generally restored, the CPCS PDU is discarded and then a transmission request is sent to the transmission side once again to recover communication.
In the conventional ATM communication system, the use efficiency of the transmission line is low when AAL5 is used for the upper layer. The reason is that when any ATM cell for which AAL5 is used has been discarded in the ATM switch unit, all the other ATM cells which form the AAL5 SAR SDU are transmitted to the terminal on the reception side nevertheless, even though this AAL5 SAR SDU is discarded in the upper layer in the reception terminal since an ATM cell is missing. This transfer of unuseful ATM cells has the possibility to cause a congestion state in the switch unit or on the transmission line at the following stage.